The present invention relates generally to telecommunications and more particularly to improving security and data transmission over fiber optic networks.
In current fiber optic networks, an electronic data stream is fed to a laser amplitude modulator. The laser amplitude modulator typically pulses or alters the laser output to create an amplitude-modulated optical signal representative of the electronic data stream. The laser amplitude modulator and laser thus define a transmitter for transmitting the optical signal over an optical fiber, which is then received by a receiver. The receiver for the amplitude-modulated optical signals of the optical data typically includes a photodiode to convert the optical signals back into the electronic data stream.
The reading of the amplitude-modulated optical data signals using a photodiode is straightforward: the optical signals either produce an electric output at the photodiode or they do not. As a result, an output electronic data stream of zeros and ones is generated.
However, optical fiber may be tapped. The optical fibers can be spliced or even merely clamped so as to obtain optical signals from the fiber. It also may be possible to tap fibers without physically touching the optical fiber, for example by reading energy emanating or dissipating along the fiber. U.S. Pat. No. 6,265,710 for example discloses such a tap. Amplitude-modulated optical signals, with their ease of detection from a photodiode, require that only a small amount of energy be tapped and passed through the photodiode in order to be converted into a tapped electronic data stream.
To confront non-secure optical and non-optical data lines, it has been known to use public key/private key encryption so that the data stream being transmitted is encoded in a format that makes it difficult to decode. Encryption however has several drawbacks, including the need for extra processing steps and time. Moreover, public key/private key encrypted data can be cracked, and the devices and algorithms for doing so are constantly improving.
In order to confront the problems of an amplitude-modulated system, U.S. Pat. No. 5,455,698 purports to disclose a secure fiber optic communications system based on the principles of a Sagnac interferometer. A data transmitter is a phase modulator for modulating counter-propagating light beams sent by a receiver round a loop. The receiver includes a light source, a beamsplitter for splitting light from the light source into counter-propagating light beams and for receiving the phase-modulated light beams, and an output detector. U.S. Pat. No. 5,223,967 describes a similar Sagnac-interferometer-based system operating over a single optical fiber.
The Sagnac-interferometer-based systems described in these patents have the disadvantage that they require the light to travel over a loop, whether back and forth in a single fiber or over a long length looped fiber. As a result, either the link budget for the single fiber must be doubled, reducing the data carrying capacity for a single fiber, or else a looped fiber with significant and expensive extra length of at least twice that of a single fiber must be laid between the transmitter and the receiver. Moreover, the receiver contains the light source, as opposed to the current installed base where the transmitter has the light source.
The Sagnac-interferometer-based systems thus are expensive to build and operate, and do not work particularly well with existing systems.
In addition, the Sagnac-interferometer-based systems in these patents desire a broadband low-coherence-length light source, for example from a light emitting diode. The light source thus typically generates light over a wavelength range of 10 nm or more.
U.S. Pat. No. 6,072,615 purports to describe a method for generating return-to-zero optical pulses using a phase modulator and optical filter. The RZ-pulse optical signal transmitted over the fiber is easily readable by a detector. The system is an amplitude-modulated system.
U.S. Pat. No. 5,606,446 purports to describe an optical telecommunications system employing multiple phase-compensated optical signals. Multiple interferometric systems are combined for the purpose of multiplexing various payloads on the same optical transmission path. The patent attempts to describe a method for providing fiber usage diversity using optical coherence length properties and a complex transmit/receive system. Each transmitter has a splitter, a plurality of fibers and a plurality of phase modulators to create the multiplexed signal, which is then demultiplexed at the receiver. This system is complex and expensive. Moreover, each phase-modulated light path is combined with a continuous wavelength base laser light path when sent over a telecommunications line, so that amplitude-modulated signals result.
As with U.S. Pat. Nos. 5,606,446, 5,726,784 discloses creating an amplitude-modulated data stream by combining a phase-modulated light path with a continuous wave base laser light path. The ""446 patent describes lasers with wavelength variance accurate to less than lnm variance to create a WDM system.
In addition, for cable systems it has been known to use a data transmission method called phase-key-shifting, or PSK. Binary PSK imparts an electric signal over a constant-wave laser output using an optical mixer. Because the fiber plant signal is a mixture of both the optical signal from the laser and the optical signal imparted by the electric signal, when the signal is received by a photodiode and passed through a low pass filter, the electrical signal can be recovered. PSK systems typically do not use a phase modulator or an interferometer.
An object of the present invention is to provide an improved security optical fiber transmission system and device. An alternate or additional object of the present invention is to provide high bandwidth optical data transport via transmission and recovery of phase-modulated optical signals. Yet another alternate or additional object of the present invention is to provide a simple yet secure phase-modulated optical data transmission system.
The present invention provides a fiber optic data transmission system comprising a transmitter having a laser emitting a continuous wave light, the transmitter including a phase modulator phase modulating the continuous wave light as a function of an electronic input data stream, so as to create a phase-modulated optical signal, an optical fiber transmitting the phase-modulated optical signal, and a receiver. The receiver includes an interferometer for receiving the phase-modulated optical signal, the interferometer having a first arm and a second arm, the second arm being longer than the first arm.
Preferably, a time delay created by the second arm with respect to the first arm matches an electronic delay imparted on the electrical input data stream. This electronic delay may be in the form of a delay in a feedback loop of an exclusive-or or exclusive-nor gate in a circuit controlling the phase-modulator.
The use of a laser for a phase-modulated telecommunications system provides many advantages over the broadband light desired by the aforementioned Sagnac systems, including that the wavelength can be selected to match the optical fiber characteristics so that the light can be transmitted further distances. Also the lasers can be used in a wave-division-multiplexing system.
The laser preferably has a wavelength such that the coherence length of the laser is longer than the second arm.
Also, a phase compensator may be provided to compensate for a phase difference different than 180 degrees that results during manufacturing of the interferometer. The phase compensator may be at the transmitter in the form of an analog or, preferably, digital summing device controlling the phase modulator, or may be a second phase modulator placed in one or more of the arms of the interferometer at the receiver.
Alternately, the interferometer can be manufactured to provide a 180-degree phase shift or difference between the two arms, as disclosed in U.S. patent application Ser. No. 09/765,153 filed Jan. 17, 2001 and Ser. No. 09/865,173 filed May 24, 2001, both of which are hereby incorporated by reference herein.
The present invention also provides an optical data transmitter including a laser for producing light, a phase modulator phase modulating the light, and an electronic control circuit for receiving electronic input data and controlling the phase-modulator, the electronic control circuit including an input data circuit having an electronic delay and a phase compensating circuit for altering an output of the input data circuit.
Preferably, the input data circuit includes a delayed-feedback exclusive-or gate. The phase compensating circuit provides a signal so that the phase modulator in the transmitter continually rotates the phase for each bit by an amount equal to [(PDxe2x88x92180)/max(Z,1)] mod 360, where PD is the phase difference in degrees imparted by the arms of the interferometer and Z is the number of bits of delay imposed by the second arm of the interferometer relative to the first arm. As used in this equation, the max() function provides the maximum of either Z or 1 as the denominator.
The present invention also provides a method for transmitting data over a fiber optic network comprising the steps of:
imparting an electronic delay in an input data stream to match an interferometric delay imposed by an interferometer in a receiver;
phase modulating a laser output using a phase modulator as a function of the electronic input data stream so as to define a phase-modulated optical data stream;
matching a phase difference imparted by the phase modulator to an interferometric phase difference imparted by the interferometer; and
transmitting the phase-modulated optical data stream over a telecommunications optical fiber.
The delay preferably is a function of the bit-rate of the electronic input data stream, for example one or two bits. A reference data stream may be interspersed between the electronic data input stream, although preferably no reference data stream is sent and the delay is a feedback-loop delay of an exclusive-or gate.
The phase difference imparted by the phase modulator may be matched to the interferometric phase difference in one of three ways or a combination thereof: (1) by manufacturing the interferometer to have a 180 degree phase difference, in which case the input electronic data is sent through an exclusive-or gate; (2) by placing an additional phase modulator in an arm of the interferometer and altering the phase difference provided by the interferometer to achieve a 180 degree phase-shift; or (3) by determining the phase difference of the interferometer and compensating for the phase-difference by supplementing the electronic input data stream with phase-compensation data.
The phase difference can be determined for method (2) or (3) above by a synchronization routine, during which a known data stream is transmitted, and the phase is rotated by the phase-compensating circuit for predetermined periods of time. When a photodiode at the output of the interferometer of the receiver reads the data stream with an acceptable error rate, the phase compensation amount set by the phase-compensation circuit is noted, and that amount of phase rotation is used for compensation. Data transmission mode can then begin.
Preferably, correction information can be sent from the receiver to the transmitter via, for example, user defined bits in a data packet sent back to the transmitter. Thus, for example, if the signal modulation depth at the receiver photodiode began to degrade, the phase-compensation circuit could slightly alter the phase imparted by the transmitter. The signal modulation depth, or alternatively modulation depth, is defined as the difference in the photodiode high level output and low level output when receiving amplitude modulated light as occurs in the described system when the phase modulated signals in each interferometer arm constructively or destructively combine at the interferometer output.